Dynamics. Note that if a collection is in progress and the task is of
the form H [e], no rule allows e to step, so no changes are required
to the semantics to ensure that e is in fact locked during collection.
Instead, we introduce a new transition rule for tasks, D-GCStep,
which allows the task to proceed with collection. The auxiliary
judgment T !GC T0 indicates that a step of garbage collection
transforms T into T0.